Thousands of Lloyds, Halifax and Bank of Scotland customers were unable to access their accounts on the first day of the new year.
The banks' mobile apps and websites were down for about half a day, sending many customers onto social media to complain about the outage.
It is understood that the disruption was caused by an internal issue and that it wasn't related to cyber security.
The three banks - all part of Lloyds Banking Group - apologised to those who were unable to log in to check their finances or make payments or transfers from the early morning hours.
After service was finally restored, a spokeswoman for the banks said: “Internet and mobile banking is now back to normal.
"We're sorry that some of our customers had issues with it this morning.”
Lloyds Banking Group did not reveal the cause of the outage.
Twitter users expressed frustrating when they realised they were locked out of their online accounts.
One wrote to Halifax: "You banking app does not currently work, a poor poor start to 2020."
Another wrote: "Why is the mobile banking app not working?!?! How long until it is back up working again."
A third added: "I couldn't sign it at 2am this morning and I still can't now.. Please sort ASAP."
Lloyds customers also complained about the outage, with one writing: "Great way to start a new year. App and site down are my payments going to go through?"
Another wrote: "Well seems that #Lloyds banking is down. Sort your s*** out guys I want to know if I can afford a #McDonalds breakfast,"
And a third added: "I need someone to call me now I want to raise a complain. My God. This is causing so much distress. I need my mobile app back on. I WANT TO RAISE A COMPLAIN."
All three banks wrote back to customers to apologise and assure them that a fix was on the way.
